ABC Fact Check took a look at the issue at the time the legislation was before the ACT Parliament, finding that its prospects of surviving a High Court challenge were doubtful.

Ultimately, the High Court found that:

The federal Parliament is able to make laws about all types of marriage, including same-sex marriage

The ACT Act and the Marriage Act were very similar

The Marriage Act is a comprehensive statement of the law of marriage

No part of the ACT Act can operate

Fact Check takes a closer look at the High Court's decision and what it means for same-sex marriage laws in Australia in the future.

The federal Parliament can make same-sex marriage laws

Under Section 51(xxi) of the Constitution, the federal Parliament has power to make laws with respect to "marriage".

Before the case was heard, some supporters of same-sex marriage argued that the federal Parliament could only make laws about "marriage" as it was understood at the time of federation (which would have been between a man and a woman). It follows that states would have the freedom to make laws about other types of marriage.

However, the High Court found:

"The status of marriage, the social institution which that status reflects, and the rights and obligations which attach to that status never have been, and are not now, immutable. Section 51(xxi) is not to be construed as conferring legislative power on the federal Parliament with respect only to the status of marriage, the institution reflected in that status, or the rights and obligations attached to it, as they stood at federation."

This decision by the court has been taken as a victory of sorts by some advocates for same-sex marriage. For example, Australian Marriage Equality national director Rodney Croome has said that "The court has removed the question mark which hung over the federal Parliament's power to pass marriage equality, re-igniting our campaign for reform at a national level". However, Fact Check's earlier work in this area indicated that most experts already believe the federal Parliament has the power to make same-sex marriage laws.

ACT law and Marriage Act very similar

When considering whether the two laws could operate at the same time, the High Court compared the ACT and Commonwealth laws and found them to be "very similar". This makes it harder for the ACT law not to encroach on the territory of the Marriage Act. The court found:

"Both Acts are... directed to the creation of a legal status deriving from the agreement of natural persons to form an enduring personal union which can be dissolved only in accordance with law and which entails legal consequences for mutual support."

The federal Marriage Act is a complete statement of the law

In its judgment, the court noted that:

"If a Commonwealth law is a complete statement of the law governing a particular relation or thing, a territory law which seeks to govern some aspect of that relation or thing cannot operate concurrently with the federal law to any extent."

One of the arguments put forward by the ACT was that because the Marriage Act only deals with opposite-sex marriage, the ACT law dedicated to same-sex marriage covers a different area and so is not inconsistent with the federal one.

However, the court's judgment leaves no doubt that the Marriage Act is to be considered a complete statement of the law relating to all types of marriage, even though same-sex marriage is not specifically ruled out. The court stated:

"The federal Parliament has not made a law permitting same sex marriage. But the absence of a provision permitting same sex marriage does not mean that the Territory legislature may make such a provision. It does not mean that a Territory law permitting same sex marriage can operate concurrently with the federal law. The question of concurrent operation depends upon the proper construction of the relevant laws. In particular, there cannot be concurrent operation of the federal and Territory laws if...the only form of marriage permitted shall be a marriage formed or recognised in accordance with [the Marriage] Act."

In the debate before the High Court decision, there had been an argument made that the Howard-era amendments to the Marriage Act, inserting a definition that marriage was between a man and a woman, in fact opened the door to state and territory same-sex marriage laws. However, the court rejected this argument, posing the question:

"Why otherwise was the Marriage Act amended... by introducing a definition of marriage in the form which now appears, except for the purpose of demonstrating that the federal law on marriage was to be complete and exhaustive?"

The decision

The court decided the whole of the ACT Act is inconsistent with the Marriage Act and so none of the provisions are operational or of any effect. The marriages that took place in the ACT under the ACT Act prior to the High Court decision no longer have any official status.

The court said:

"Because the ACT Act does not validly provide for the formation of same sex marriages, its provisions about the rights of parties to such marriages cannot have separate operation and are also of no effect."

The court also ordered that the ACT Government pay the Commonwealth's costs.

Where to from here?

As noted in earlier fact checks on the issue, the weight of expert opinion was against the ACT Act in its present form. However, some experts believed that a differently worded law might have a better chance of resisting a High Court challenge.

The High Court has now to all intents and purposes resolved that point. By deciding that "the kind of marriage provided for by the [Marriage] Act is the only kind of marriage that may be formed or recognised in Australia", the High Court has in essence ruled out state laws being used to bring in same-sex marriage. The same-sex marriage issue is one that will have to be decided in the federal Parliament.